This disclosure relates generally to a sheet feeder for use in a printing machine, such as an electrophotographic reproduction machine. More particularly the disclosure concerns use of a vacuum to remove sheets from a stack and transfer the sheets to the imaging portion of the electrophotographic reproduction machine.
In one type of electrophotographic printing or reproduction machine, such as the machine M shown in FIG. 1, a photoconductive member or belt 10 is charged by a corona generating device 12 at a station A to a substantially uniform potential so as to sensitize the surface thereof. At a station B, the charged portion of the photoconductive member 10 is exposed to a light image of an original document being reproduced obtained from a scanning device, such as a raster output scanner 14. Exposure of the charged photoconductive member selectively dissipates the charges thereon in the irradiated areas which records an electrostatic latent image on the photoconductive member corresponding to the informational areas contained within the original document.
After the electrostatic latent image is recorded on the photoconductive member, the latent image is developed by bringing a developer material into contact therewith at a series of developer stations C and D. Generally, the developer material comprises toner particles adhering triboelectrically to carrier granules. The toner particles are attracted from the carrier granules to the latent image forming a toner powder image on the photoconductive member. The toner powder image is then transferred from the photoconductive member to a copy sheet. The toner particles are heated to permanently affix the powder image to the copy sheet. For a typical black and white electro-photographic printing machine, a single development station C may be provided. On the other hand, with the advent of multicolor electrophotography, multiple additional development stations D may be provided that fix color toner to the photoconductive member 10.
Subsequent to image development, a sheet S of support material is moved into contact with the toner images at a transfer station G. At this station, a transfer dicorotron 16 sprays positive ions onto the backside of the sheet S which attracts the negatively charged toner particle images from the photoreceptor 10 to the sheet S. A detack corotron 18 is provided for facilitating stripping of the sheet S from the surface of the photoreceptor. After transfer, the sheet S travels to a fusing station H where a heated fuser roller assembly 20 permanently affixes the toner powder to the sheet S.
Many high speed color printers, such as those described above, are designed to feed a wide variety of sheet types for various printing jobs. Customers demand that such a machine be able to image on stock having different dimensions, a wide range of paper weights, and appearance characteristics that vary from rough flat appearing sheets to very high gloss coated paper stock. Each of these sheet types and size has its own unique characteristics and in many instances present very different problems associated the high speed feeding of the sheet types to the imaging section of the machine.
There is shown in FIG. 2, a side elevational schematic view of a high speed sheet feeder, generally indicated by reference numeral 200. The basic components of the feeder 200 include a sheet support tray 210, which may be tiltable and self adjusting to accommodate various sheet types and characteristics; multiple tray elevator mechanisms 220, 230; a vacuum shuttle feedhead 300; a lead edge multiple range sheet height sensor 340; a multiple position stack height sensor 350; a variable acceleration take a way roll (TAR) 400; inboard and outboard sheet fluffers 360, and trail edge fluffer 362.
The feedhead is a top vacuum corrugation feeder (VCF), so distance control of the top sheets in the stack T from the acquisition surface 302 and the fluffer jets 360 and 362 are very important. The acquisition surface 302 is the functional surface on the feed head 300 or vacuum plenum. The two sensors 340, 350 together enable the paper supply to position the stack T. The multi-position stack height sensor 350 contacts the sheet stack T to detect two or more specific stack heights. This sensor 350 works in conjunction with the second sensor 340 near the stack lead edge which also senses the distance to the top sheet, but without sheet contact. The two sensors together enable the paper supply to position the stack T with respect to an acquisition surface 302 of the feedhead 300, both vertically and angularly in the process direction. This height and attitude control greatly improves the capability of the feeder to cope with a wide range of paper basis weight, type, and curl.
The paper feeder 300 acquires individual sheets S of paper (using air pressure) from the top of a stack T and transports them forward to the TAR 400. Among the independent variables in the paper feeder design are three sets of air pressures, including air knife pressure and fluffer pressures that supply air for sheet separation and vacuum pressure that causes sheets to be acquired by the shuttle feed head assembly. Each set of pressures is supplied from one combination blower or may be supplied by an independent blower. As fluffer pressure increases the sheets on the top of the stack become more separated with the top most sheets being lifted closer to the vacuum feed head. As the fluffing pressure gets higher, the risk of more than one sheet being moved into the take-away nip as the feed head moves also increases, (a.k.a. multifeed). As the fluffing pressure gets lower, the risk of the top sheet not getting close enough to the feed head (and thus not becoming acquired by the vacuum present on the bottom of the feed head) increases. This failure results in no sheet being fed when the feed head moves forward, (a.k.a., misfeed or late acquisition). The optimum amounts of fluffer and vacuum feed-head pressures are a function of the size and weight of the sheets (larger, heavier sheets requiring more fluffing and vacuum and visa-versa for smaller, lighter sheets).
During each sheet feed, when the trailing edge of the sheet passes the stack height arm 352 (FIG. 3), the arm compresses the stack T, the stack height sensors 340, 350 measure the position of the solid stack, and the stack height arm 352 is raised again after about 25 ms. The timing of the movement of the arm is controlled by a cam 348 that is driven by a stepper motor 310. Once the trailing edge of the sheet S passes the position of the lead edge sensor 340, the position of the leading edge of the fluffed stack T is measured. The values of these measurements are then compared to the desired states for the paper being fed and the tray is adjusted accordingly. The fluffer jets 360, 362 remain activated during these steps.
The feed head 300 is a top vacuum corrugation feeder which incorporates an injection molded plenum/feed head 301 with a sheet acquisition and corrugation surface 302, as shown in FIGS. 2 and 3. The feed head 300 is optimally supported at each corner by a ball bearing or other low friction roller/track assembly 304. In a typical installation, the feed head 300 is driven forward twenty mm and returned twenty mm to its home position by a continuous rotation and direction twin slider-crank drive 346 mounted on the double shaft stepper motor 310. This feed head travel includes five mm of over-travel to account for paper loading tolerance and misregistration. This drive results in a linear sheet speed of about 420 mm/s as the sheet is handed off to the take away roll 400 (TAR). The TAR 400 may also be stepper driven to accelerate the sheet S up to transport speed. The feed head 300 supports each sheet fully as it is carried to the TAR 400. This approach avoids a “pushing on rope” syndrome that plagued earlier systems.
Thus, the prior sheet feed apparatus 300 includes a vacuum source, the vacuum source being selectively actuatable to acquire and release a top sheet from a stack; a feed head that is attached to the vacuum source to acquire the top sheet of the stack; and a unidirectional drive mechanism that is driven in a single direction to cause the feed head to reciprocate from a first position to a second position. Additionally, the sheet feed apparatus can include a stack height sensor actuator coupled to the unidirectional drive mechanism and a stack height sensor attached to the stack height sensor actuator so that the stack height sensor contacts and disengages the sheet stack at a preselected time coordinated with the reciprocating motion of the feedhead. Moreover, the stack height sensor actuator may comprise a cam member that is attached to the unidirectional drive mechanism and rotating therewith; a biasing member; a cam follower that is attached to the biasing member and biased into contact with said cam and attached to said stack height sensor to control the movement of said stack height sensor. Furthermore, the sheet feed apparatus may include a unidirectional drive mechanism which comprises a stepper motor operating in a unidirectional rotational mode.
In these prior feeder mechanisms, the entire sheet feed apparatus 300 is propelled by the motor 310. Thus, the motor must be powerful enough to accurately and precisely move the apparatus 10 in order to transport a single sheet to the TAR 400. Such a motor is relatively expensive, generates a fair amount of heat and requires a relatively significant amount of energy to operate. Moreover, driving the entire sheet feeder mechanism imposes a limit on feed speed arising from the inertia of the mechanism 300, and increases the risk of skewing the acquisition surface 302 and, ultimately, the sheet S as it is received by the TAR. These limitations of previously known sheet feeders are addressed by the integrated slide feeder set forth in co-pending U.S. patent application entitled “Integrated Vacuum Slide Feeder” that was filed on Sep. 20, 2005 and assigned Ser. No. 11/230,961.
In some previously known sheet feeders, the sheet feed apparatus includes a vacuum box that includes vertically displaceable skirts. The vertically displaceable skirts move downwardly under the force of gravity to lie proximate the top sheet of the stack. The interior of the vacuum box formed by the skirts is in fluid communication with a vacuum source to generate a negative pressure within the interior of the box. This negative pressure helps attract the top sheet towards the lower edge of the box. The stiffness of the paper moves the skirts vertically upwardly as the paper moves in response to the negative pressure inside the vacuum box. To facilitate the separation of the lower sheets from the top sheet, an air knife stream may be directed at the lead edge of the sheets being held above the stack by the negative pressure. Corrugation molded into the acquisition surface of the vacuum plenum produces “gaps” between the top acquired sheet and the second sheet. Sheets are separated either by fluffing them with air or by acquiring and corrugating them. Thus, the vacuum skirts helps lift one or more sheets from the stack so that the air knife stream and/or gravity may act on the lower sheets to separate them from the topmost sheet.
Attaching the displaceable skirts to the paper transport subsystem is relatively expensive from a manufacturing point of view. The attachment requires the use of retainers and fasteners. Burrs on the displaceable skirts may cause the skirts to bind during their vertical travel with resulting media sheet misfeeds. Additionally, air knife pressure may also cause the skirts to bind during their vertical movement. Hence, installation of the displaceable skirts is time consuming and adds expense to the manufacture of image transfer systems. Additionally, the operation of previously known displaceable skirts may not be reliable.